"Stroking" Neurons

We have many different types of neurons within our peripheral somatosensory system. In addition to basic mechanoreceptors, we have neurons corresponding to pain sensations, and channels that are temperature sensitive. However, one phenomenon that was not explained at the neuronal level until recently, is the sensation of stroking. On the behavioral level, we know that stroking or grooming is pleasurable in such phenomenon as maternal care. But how is this transduced at the molecular level?

Researchers in David Anderson’s lab at Caltech recently discovered a class of neurons that selectively responds to “massage-like” stimulations. Experiments were performed in-vivo to directly measure the effect of certain stimulations. Calcium imaging, a type of imaging designed to study activity of neurons, was used in the spinal cord, where the cell bodies of neurons projecting to the periphery are located. After mice were pinched, poked, and light-touch stroked on their paws, the researchers found that a subset of neurons was selectively activated to only the light-touch stimulus.

Mouse being stroked (Discover Magazine and David Anderson Lab)

To help support the results behaviorally, mice were given a two-choice test between a chamber where they received a drug activating the light-touch neurons, or a chamber where they received a control saline solution. Mice preferred the chamber with the drug that activated the light-touch neurons, suggesting that the animals form a positive association with having these neurons activated.

While similar neurons are thought to exist in humans, more studies need to be done on the nature of the potential light-touch fibers. These studies, when paired with behavioral data, can also provide insight into the biological basis of stroking and grooming in behaviors such as maternal care or social bonding experiences. Perhaps the reasons for this innate behavior (who ever thought of hugs as a feel-good mechanism, anyway?) actually has a stronger molecular link than we initially thought.